1. Field of the Invention
The present invention relates to a method and an apparatus for controlling transmission power in a wireless communication system, and more particularly, to a method and an apparatus for controlling transmission power in a wireless communication system capable of decreasing handover failure probability at a cell boundary in a linear cellular communication system in which each base station uses an unidirectional beam.
2. Description of Related Art
A cellular wireless communication system includes a plurality of base stations disposed at a predetermined distance to support a wireless access of user terminals distributed in a space. The user terminal may have mobility, and therefore a handover for changing an access from a currently accessed base station to neighboring base stations is required. The handover in a current cellular wireless communication system is performed to access a base station having large signal strength measured based on a value obtained by allowing a user terminal to measure signal strength from a base station. For example, in a long term evolution (LTE) system, the handover is carried out using measurement values such as reference signal received power (RSRP) and reference signal received quality (RSRQ).
The handover based on the signal strength is performed when the signal strength of the neighboring cells is larger than that of the current cell by a predetermined value or more as a result of comparing the value of the signal strength of the current cell with the value of the signal strength of the neighboring to cells. Since the further away from the base station, the smaller the signal strength, both of the signal strength of the current cell and the signal strength of the neighboring cells are considerably decreased at the cell boundary area where the handover is carried out. Therefore, a process of comparing signal strength between attenuated signals is performed.
To support a wireless access to a terminal installed in a train or a vehicle moving along a defined path, a linear cellular wireless communication system may be used. Unlike the typical wireless communication system, the linear cellular wireless communication system needs to support a wireless access only in a linear space having a narrow width and therefore the base stations are disposed in a row at a predetermined distance along a rail or a road. In this case, a directional antenna is applied to the base station to generate a narrow beam width, thereby concentrating a signal on a rail or a road. At this point, the respective base station antennas may generate a directional beam in either a moving direction of a terminal or an opposite direction to the moving direction thereof. In this case, the terminal needs to process only a beam received in a predetermined direction even when moving between the base stations and therefore is advantageous in signal processing and radio frequency (RF) processing associated with mobility.
However, in the case of the linear cellular wireless communication system in which the respective base stations use a unidirectional beam, there arises a problem in that a difference between a value of signal strength of a serving cell which is measured for the handover and a value of signal strength of neighboring cells is considerably large. For example, as a train approaches to any one of base stations installed along a rail, signal strength received by a terminal from the corresponding base station is increased, and as a result the received signal strength becomes a very large value. However, if a train passes through the corresponding base station, the terminal receives a signal from a base station at the next location, and since the base station at the next location is located at a far distance from the terminal, the signal strength received by the terminal from the base station at the next location becomes a very small value. As described above, in the case of the linear cellular wireless communication system, since the received signal strength of the cell is suddenly changed from a very large value to a very small value or from a minimum value to a maximum value, the difference between the signal strength of the serving cell and the signal strength of the neighboring cells becomes very large and the difference between the two signal strengths is very large, such that if the two signals do not enter a dynamic range of a transceiver, the signal of the serving cell and the signal of the neighboring cells may not be measured simultaneously. Therefore, it is difficult to apply the existing method for determining whether to perform the handover by simultaneously measuring the signal strength of the serving cell and the signal strength of the neighboring cell.